This proposal represents a continuation of our studies on aflatoxins and cytochromes P-450. The focus of the studies is on the initial steps of aflatoxin B1 (AFB1) carcinogenesis including: (a) activation and deactivation of AFB1 by various cytochromes P-450; (b) elucidation of the genetic regulation of these cytochromes P-450; (c) investigation of the genetic and molecular mechanisms involved in the repression of the cytochromes during multistage hepatocarcinogenesis; and (d) investigation of the molecular and genetic bases of the relationship between cytochromes P-450 repression and induction of hepatocarcinogenesis by AFB1. Various cytochromes P-450 involved in the preferential metabolism of AFB1 via activation and deactivation pathways will be isolated to develop molecular probes (cDNA, genomic DNA). These molecular probes will be characterized and used to elucidate: (a) molecular basis of regioselectivity of various cytochromes P-450 in the metabolism of AFB1 via various pathways; and (b) genetic and molecular alterations required to induce cytochromes P-450 repression during multistage hepatocarcinogenesis. Much of the work will be performed in vivo in rats and in vitro with labeled aflatoxins, bacterial mutagenesis systems, RNA, DNA, and cytochromes P-450 isolated from rats. Particular methods to be used include: radiochemical methods, high pressure liquid chromatography, biochemical and enzymologic methods, ultra-centrifugation, hybridoma and recombinant DNA techniques.